1. Field of the Invention:
The present invention relates to the preparation of dinitrotoluenes, and, more especially, to the preparation of dinitrotoluenes by reacting at least one mononitrotoluene with nitric acid, in liquid phase, in the presence of an acidic solid.
2. Description of the Prior Art:
Dinitrotoluenes, in particular 2,4-dinitrotoluene, mixed, if appropriate, with 2,6-dinitrotoluene, are useful intermediates in the production of toluene diisocyanate (TDI) for the ultimate preparation of various polyurethanes. 2,4-Dinitrotoluene is therefore produced industrially and on a very large scale. In such a context, the importance of an increase of a few percent in the degree of conversion and/or selectivity, for example, in any of the stages of the process is readily apparent.
2,4-Dinitrotoluene is conventionally produced by a liquid phase double nitration of toluene with a mixture of nitric acid and concentrated sulfuric acid. The concentration of the sulfuric acid employed for the second nitration (nitration of mononitrotoluene to dinitrotoluene) is typically on the order of 98%. Notwithstanding the problems of corrosion and the high cost of recycling large amounts of sulfuric acid, such process presents major other disadvantages, namely, the problematic nature of the removal of the catalyst, which is generally solved by repeated washings with water, and the requirement to concentrate the sulfuric acid before it is recycled, insofar as the reaction itself produces water.
Therefore, serious need continues to exist in this art for an efficient process for the preparation of dinitrotoluene, enabling the aforementioned disadvantages to be at least partially prevented.